Power tools such as drills, grinders, cutters, hammers, sanders, pressure washers, foam guns, routers, hole diggers, and winches etc, typically comprises a motor which transfers torque to a tool via a transmission. Often, the transmission includes a gear. Planet gearing is sometimes referred to as “Epicyclic gearing” and describes a gear system with a housing comprising one or more planet wheels rotating about a centrally located sun gear. Sometimes, the planet wheels are mounted on a movable carrier. The carrier may either be fixed relative to the housing, or it may rotate relative to the housing and/or relative to the sun gear.
The gear system may further incorporate an outer ring gear with radially inwardly projecting gear teeth, generally referred to as the annulus. The annulus meshes with the planet wheels and at least one of the planet wheels again mesh with the sun gear.
There are several ways in which an input rotation can be converted into an output rotation. In general, one of the above mentioned basic components, i.e. the sun, the carrier or the annulus, is held stationary; one of the two remaining components is an input, providing power to the system, while the last component is an output, receiving power from the system.
The ratio of input rotation to output rotation depends upon the number of teeth in each gear included in the system and depends further upon which component is held stationary. When e.g. the carrier is held stationary, and the sun gear is used as input, the planet wheels simply rotate about their own axes at a rate determined by the number of teeth in each gear. If the sun gear has S teeth, and each planet wheel has P teeth, the ratio is equal to S/P. If the annulus has A teeth, the planet wheels drive the annulus in a ratio of P/A turns for each turn of the planet wheels.
In one implementation of a planet gear system, the annulus is held stationary and the sun gear is used as the input. This provides the lowest gear ratio, i.e. 1/(1+A/S), attainable with a planet gear train.
In the gear system mentioned in the introduction, the double-ring planet gear, i.e. the planet wheels with at least two concentrically joined gear wheels, comprises a first gear ring which is integral with a second gear ring and the primary and secondary annulus gears are typically formed internally in disc shaped gear members which thereby form housing for the gear system. This gear system offers a particularly low gear ratio at relatively small outer dimensions of the gear system and it is therefore applied in mechanical system with narrow space, e.g. for electrical operation of a rear-view mirror in a vehicle.
Often, power tools are provided with a gear mechanism which can either change between different gear ratios or which can release and thus prevent transmission of torque at a specific counter torque, i.e. the function known from a torque wrench etc. In traditional transmission systems, the rotor rotation is normally limited or stopped by a key which locks the output of the gearbox directly to the rotor when stopping of the rotor is intended. This is typically seen as a safety feature which prevents undesired rotation of the rotor when the tool is stopped. The traditional way of locking the output directly to the rotor is a common feature in many handheld grinders etc. Consequently the key needs to be able to withstand the entire torque applied to or by the rotor. This increases the demands for mechanical strength in the transmission and braking system thus increases costs, weight, and dimensions of the power tool—features which are very essential particular with regards to hand-held power tools. A traditional solution within servo systems and other non-power tool related applications to limit or stop rotation of the rotor by braking is to connect the input shaft of the gearbox to the frame, so that the advantage of the full gear ratio helps in stopping or limiting the rotor rotation, i.e. the torque conversion helps in braking the rotor, and the braking system therefore need not to be as strong as required in traditional power tools. This solution, however, is complicated in relation to a power tool, where it is typically desired to have the transmission as one unit being separate from the motor, and where the motor is typically a low cost standard motor without braking means.